内蒙古甲乌拉铅锌银矿区岩浆岩年代学、地球化学特征及其对成矿的约束

The zircon U-Pb geochronology and the geochemistry of magmatic rocks and their constraints on the mineralization of Jiawula Pb-Zn-Ag deposit,Inner Mongolia,China

内蒙古满洲里地区的甲乌拉铅锌银矿区的成岩、成矿事件及其相互关系目前仍有很大争议。文中对矿区内与成矿密切相关的石英斑岩、闪长玢岩开展详细研究。锆石SHRIMP U-Pb年代学研究表明:矿区致矿侵入体之一石英斑岩形成于(129±4.8)Ma,成矿晚(或后)期的闪长玢岩形成于约(124±2.5)Ma;认为铅锌银等成矿作用应发生于(129±4.8)^(124±2.5)Ma,为早白垩世末期。岩石地球化学特征显示石英斑岩具有高硅、高钾、贫钙等特征,TAS图上属于亚碱性系列,硅-铁镁图上属于钙碱系列,硅钾图上属于高钾钙碱系列。轻重稀土分馏明显,具有明显的负铕异常。相对富集大离子亲石元素(LILE),而亏损高场强元素(HFSE),且具有低Sr、高Y等特征。闪长玢岩同样展现出高钾、高铝、贫钙特征,TAS图上属于亚碱性系列,硅铁镁图上属于拉斑系列;轻、重稀土分馏明显,具明显负铕异常,但稀土总量较高;相对富集大离子亲石元素(LILE),亏损高场强元素(HFSE),同具有低Sr、高Y等特征。这些研究结果表明:石英斑岩具有火山弧的特征,主要来源于壳源的熔融;闪长玢岩具有活动大陆边缘弧的特征,主要来源于地幔的熔融,但经历了壳源的混染上述岩浆与甲乌拉的铅锌银矿;推测应为蒙古—鄂霍次克洋由俯冲向陆陆转换阶段产物。

The petrogenic-metallogenic events developed in Jiawula Pb-Zn-Ag deposit and their relationships have been highly controversial. This paper reports the SHRIMP zircon U-Pb geochronology of the Jiawula ore- forming quartz-porphyry（JQP） and the late metallogenic dioritic-porphyrite （JDP）, and their geochemical characteristics. JQP was formed at about 129--4-4. 8 Ma and JDP at about 124-1-2. 5 Ma. Therefore, it is reseanahle to consider that the Jiawula Pb-Zn-Ag mineralization should be occurred during 129 - 124 Ma, i.e. the end of Early Cretaceous. The JQP rocks show high Si, K and low Ca, and belong to sub alkaline series in the TAS diagram, to CA series in the diagram of SiO2-TFeO/MgO, and to high potassium calc-alkalic series in the diagram of SiO2-K2 O, enriched in LILE, depleted in HFSE, and having low Sr and high Yb concentrations. The JQP rocks show high Si, K and low Ca, and belong to sub-alkaline series in the TAS diagram and to TH series in the diagram of SiO2-TFeO/MgO, enriched in LILE, depleted in HFSE, and having low Sr and high Yb concentrations. The geochemical characteristics indicate that the JQP rocks are formed in continental volcanic arc setting with magma derived from crustal melting and the JDP rocks are formed in active continental margin arc setting with magma derived from mantle melting. So this paper suggests that the JQP, JDP magmatism and Pb-Zn-Ag mineralization in Jiawula all are the products of transformation of Mongolia-Okhotsk Ocean from subduction to continenl-continent collision at Early Cretaceous time. The Pb-Zn-Ag deposit in Jiawula should be formed by the post-supereruption metallogenesis.